EP3467129A1 - Welding joint having excellent fatigue characteristics and method for manufacturing same - Google Patents
Welding joint having excellent fatigue characteristics and method for manufacturing same Download PDFInfo
- Publication number
- EP3467129A1 EP3467129A1 EP17806952.2A EP17806952A EP3467129A1 EP 3467129 A1 EP3467129 A1 EP 3467129A1 EP 17806952 A EP17806952 A EP 17806952A EP 3467129 A1 EP3467129 A1 EP 3467129A1
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- EP
- European Patent Office
- Prior art keywords
- welding joint
- welded metal
- welding
- remolten
- defocused laser
- Prior art date
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- 238000003466 welding Methods 0.000 title claims abstract description 112
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 26
- 238000000034 method Methods 0.000 title claims abstract description 23
- 239000002184 metal Substances 0.000 claims abstract description 46
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 41
- 239000010959 steel Substances 0.000 claims abstract description 41
- 239000002344 surface layer Substances 0.000 claims abstract description 17
- 230000001678 irradiating effect Effects 0.000 claims description 12
- 229910001563 bainite Inorganic materials 0.000 claims description 4
- 229910000734 martensite Inorganic materials 0.000 claims description 4
- 230000000052 comparative effect Effects 0.000 description 23
- 230000003287 optical effect Effects 0.000 description 17
- 230000004886 head movement Effects 0.000 description 7
- 239000000463 material Substances 0.000 description 7
- 239000011324 bead Substances 0.000 description 5
- 230000003252 repetitive effect Effects 0.000 description 4
- 238000009826 distribution Methods 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 230000001276 controlling effect Effects 0.000 description 2
- 238000000151 deposition Methods 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 239000013077 target material Substances 0.000 description 1
- 239000010409 thin film Substances 0.000 description 1
- 238000010792 warming Methods 0.000 description 1
Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/02—Seam welding; Backing means; Inserts
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/23—Arc welding or cutting taking account of the properties of the materials to be welded
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D10/00—Modifying the physical properties by methods other than heat treatment or deformation
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D10/00—Modifying the physical properties by methods other than heat treatment or deformation
- C21D10/005—Modifying the physical properties by methods other than heat treatment or deformation by laser shock processing
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2103/00—Materials to be soldered, welded or cut
- B23K2103/02—Iron or ferrous alloys
- B23K2103/04—Steel or steel alloys
Definitions
- the present disclosure relates to a welding joint and a method for manufacturing the same, and more particularly, to a welding joint having excellent fatigue characteristics, subjected to local remelting by irradiating a defocused laser thereon, and a method for manufacturing the welding joint.
- an angle also called a toe angle
- an end also called a toe portion of a welding joint which is a stress concentrator.
- controlling the material and stress of the toe portion is also an important factor.
- Prior document has proposed a method of continuously hitting the ends of welding beads with a chipper (hitting pin) to form plastic-deformed areas, thereby improving fatigue characteristics by providing compression stress.
- a target material may be deformed after peening. Therefore, applying them to a thin-film steel for vehicle is limited, and also, upon application to local areas such as the ends of welding beads, a defect such as micro-crack may be made in the welding beads having relatively high brittleness.
- Patent Document 0001 Japanese Laid-Open Patent Application No. 2014-004609 (Laid-open on January 16, 2014)
- the embodiments of the present disclosure are directed to providing a welding joint having excellent fatigue characteristics.
- the embodiments of the present disclosure are directed to providing a method for manufacturing a welding joint having excellent fatigue characteristics, subjected to local remelting by irradiating a defocused laser thereon.
- One aspect of the present disclosure provides a welding joint having excellent fatigue characteristics, including: a welded metal 20 formed by welding between a plurality of steel sheets 10 arranged in such a way to overlap each other at a connecting portion between the plurality of steel sheets 10; and a remolten portion 30 which is formed at an end of the welded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater.
- the remolten portion 30 may be formed by irradiating a defocused laser 1 on the end of the welded metal 20 to remelt the end of the welded metal 20.
- the surface layer of the remolten portion 30 may have a bainite or martensite structure.
- a toe angle ⁇ of the remolten portion 30 may be 30° or smaller.
- fatigue strength of the welding joint may be 260 MPa or greater.
- fatigue strength of the welding joint may be 70% or more of fatigue strength of the steel sheets 10.
- Another aspect of the present disclosure provides a method for manufacturing a welding joint, including operation of irradiating a defocused laser 1 on an end of a welded metal 20 formed by welding a connection portion between a plurality of steel sheets 10 arranged in such a way to overlap each other at the connecting portion to remelt the end of the welded metal 20.
- an output of the defocused laser 1 may be 6 kW, and movement speed of the defocused laser 1 may be 6 m/min.
- a defocus length L of the defocused laser 1 may be 15 to 20 mm.
- the defocus of the defocused laser 1 may cover the end of the welded metal 20 and a part of a heat affected zone (HAZ) 11 of the steel sheets 10 adjacent to the end of the welded metal 20.
- HZ heat affected zone
- an irradiation angle of the defocused laser 1 may be 30 to 45°.
- the concentration of stress may be mitigated to improve fatigue strength of the welding joint.
- the fatigue strength of the welding joint may be improved.
- a welding joint having excellent fatigue characteristics may include: a welded metal formed by welding between a plurality of steel sheets arranged in such a way to overlap each other at a connecting portion between the plurality of steel sheets; and a remolten portion which is formed at an end of the welded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater.
- FIG. 1 is a plan view for describing a welding joint having excellent fatigue characteristics, according to an embodiment of the present disclosure.
- FIG. 2 is a cross-sectional view of the welding joint shown in FIG. 1 , taken along line I-I' of FIG. 1 .
- FIG. 3 is a cross-sectional view for describing a method for manufacturing a welding joint according to an embodiment of the present disclosure.
- FIG. 4 is a picture of a cross section of a welding joint according to an embodiment of the present disclosure, photographed by an optical microscope.
- FIG. 5 is a graph showing a hardness distribution of a welding joint according to an embodiment of the present disclosure.
- FIGS. 1 to 5 a welding joint according to an embodiment of the present disclosure, and a method for manufacturing the same will be described in detail.
- the welding joint having the excellent fatigue characteristics may include a welded metal 20 formed by welding between a plurality of steel sheets 10 arranged in such a way to overlap each other at a connecting portion between the plurality of steel sheets 10, and a remolten portion 30 which is formed at an end of the welded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater.
- the end of the welded metal 20 may be an area of the welded metal 20 adjacent to the steel sheets 10.
- the steel sheets 10 may be high-strength hot-rolled sheet steels having tensile strength of 780 MPa or higher and a thickness of 6 mmt or smaller.
- tensile strength 780 MPa or higher
- a thickness of 6 mmt or smaller the durability of a product made of such a high-strength steel needs to be secured.
- an angle also called a toe angle
- an end also called a toe portion
- a stress concentrator controlling the material and stress of the toe portion may be also an important factor.
- the remolten portion 30 may be formed at the end of the welded metal 20.
- the remolten portion 30 may be formed by irradiating a defocused laser on the end of the welded metal 20 to remelt the end of the welded metal 20.
- the end of the welded metal 20, that is, the toe portion may be remolten through heat treatment using a defocused laser, and accordingly, the remolten portion 30 may have a toe angle ⁇ of 30° or smaller.
- toe angle ⁇ of the remolten portion 30 exceeds 30°, stress may be concentrated on the remolten portion 30, so that the fatigue strength is reduced to 100 MPa or smaller, resulting in a reduction of a fatigue life.
- the radius of curvature of the remolten portion 30 may increase. Accordingly, since the radius of curvature of the remolten portion 30 acting as a repetitive fatigue stress concentrator increases, the concentration of stress onto the remolten portion 30 may be mitigated.
- the radius of curvature of the remolten portion 30 may be greater than the radius of curvature of the welded metal 20.
- a surface layer of the remolten portion 30 may be partially hardened.
- the defocused laser can minutely control its irradiating area, remelting may be locally generated, compared with cases in which different kinds of heat sources are used, thereby minimizing softening of a HAZ 11 according to remelting.
- a Vickers hardness value of the surface layer may be 200 HV or greater.
- the surface layer of the remolten portion 30 may mean an area within 800 ⁇ m from the surface of the remolten portion 30.
- the Vickers hardness value of the surface layer of the remolten portion 30 is smaller than 200 HV, the welding joint may be easily fractured due to repetitive fatigue stress.
- FIG. 5 is a graph showing a hardness distribution of the welding joint. Referring to FIG. 5 , it is confirmed that a hardness value of the surface layer of the remolten portion 30 is equal to or greater than 200 Hv.
- the surface layer of the remolten portion 30 subject to the heat treatment may have a bainite or martensite structure.
- a bainite or martensite structure may appear within 800 ⁇ m from the surface of the remolten portion 30.
- the fatigue strength of the welding joint may be 260 MPa or greater.
- the fatigue strength of the welding joint may be 70% or more of the fatigue strength of the steel sheets 10. That is, by securing the fatigue strength of the welding joint to a level that is maximally similar to that of a steel sheet that is a base material, an application range of a high-strength hot-rolled sheet steel for thinning and weight lightening of components such as vehicle chassis components may be expanded.
- the welding joint according to an embodiment of the present disclosure may be manufactured through operation of irradiating a defocused laser on an end of the welded metal 20 formed by welding a connecting portion between the plurality of steel sheets 10 arranged in such a way to overlap each other at the connecting portion to remelt the end of the welded metal 2.
- a defocused laser 1 may irradiate a laser on the end of the welded metal 20.
- the defocused laser 1 may be a CO 2 solid-state laser.
- the output of the defocused laser 1 may be limited to this, and for example, the output of the defocused laser 1 may be 6 kW.
- head movement speed of the defocused laser 1 may be 4 to 6 m/min.
- the head movement speed of the defocused laser 1 When the head movement speed of the defocused laser 1 is lower than 4 m/min, the end of the welded metal 20, that is, the surface layer material of the remolten portion 30 may be not easily hardened. When the head movement speed of the defocused laser 1 is higher than 6 m/min, a sufficient melting rate for remelting at the end of the welded metal 20 may be not secured so that a desired toe angle of the remolten portion 30 may be not secured.
- a defocus length L of the defocused laser may be 15 to 20 mm.
- the defocus of the defocused laser may cover the end of the welded metal 20 and a part of the HAZ 11 of the steel sheets 10 adjacent to the end of the welded metal 20.
- the defocus length L of the defocused laser is shorter than 15 mm, there is a problem that the defocused laser cannot remelt all of the end of the welded metal 20 and the HAZ 11 of the steel sheets 10 formed by welding.
- the defocus length L of the defocused laser is longer than 20 mm, there is a problem that a heat input for remelting is insufficient.
- an irradiation angle of the defocused laser may be 30 to 45°.
- the irradiation angle of the defocused laser When the irradiation angle of the defocused laser is smaller than 30°, a sufficient melting rate for remelting at the end of the welded metal 20 may be not secured so that a desired toe angle of the remolten portion 30 may be not secured. When the irradiation angle of the defocused laser is greater than 45°, a sufficient melting rate may be not secured at a lower steel sheet 10A of the overlapping joint so that the surface layer material may be not easily hardened.
- FIG. 6 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to an embodiment of the present disclosure, photographed by an optical microscope.
- FIG. 7 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to an embodiment of the present disclosure, photographed by an optical microscope.
- FIG. 8 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope.
- FIG. 9 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope.
- FIG. 10 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope.
- FIG. 6 is a picture of the cross section of a welding joint according to the embodiment 1, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the embodiment 1 are measured and written on Table 1 below.
- FIG. 7 is a picture of the cross section of a welding joint according to the embodiment 2, photographed by an optical microscope.
- the manufacturing conditions and fatigue strength of the welding joint according to the embodiment 2 are measured and written on Table 1 below.
- FIG. 8 is a picture of the cross section of the welding joint according to the comparative example 1, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the comparative example 1 are measured and written on Table 1 below.
- FIG. 9 is a picture of the cross section of the welding joint according to the comparative example 2, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the comparative example 2 are measured and written on Table 1 below.
- FIG. 10 is a picture of the cross section of the welding joint according to the comparative example 3, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the comparative example 3 are measured and written on Table 1 below.
- Table 2 is a table for describing a method for measuring the fatigue strength of the welding joint according to the embodiment 2.
- the welding joint is not fractured even at the repetition number of 2,000,000 cycles under the maximum load of 25.5kN, and at this time, the maximum strength is 273 MPa.
- the welding joint is fractured at 92,586 cycles under the maximum load of 26.0 kN increased by 0.5 kN. At this time, the maximum strength is 277 MPa. Accordingly, the fatigue strength of the welding joint according to the embodiment 2 is 273MPa.
- Table 3 is a table for describing a method for measuring the fatigue strength of the welding joint according to the comparative example 4.
- the welding joint is not fractured even at the repetition number of 2,000,000 cycles under the maximum load of 20.0 kN, and at this time, the maximum strength is 217 MPa. Accordingly, it is confirmed that the fatigue strength of the welding joint according to the comparative example 4 is 217 MPa.
- the welding joint according to an embodiment of the present disclosure secures fatigue strength of 260 MPa, an application range of a high-strength hot-rolled sheet steel for thinning and weight lightening of components such as vehicle chassis components may be expanded.
- a welding technique including the welding joint having excellent fatigue characteristics may be used in various fields such as industrial sites.
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Abstract
Description
- The present disclosure relates to a welding joint and a method for manufacturing the same, and more particularly, to a welding joint having excellent fatigue characteristics, subjected to local remelting by irradiating a defocused laser thereon, and a method for manufacturing the welding joint.
- To meet the fuel efficiency regulatory policy for environmental protection to deal with global warming problem, etc. in the vehicle field, technical studies for weight lightening of vehicle bodies and components are emerging as a great issue. Along with this trend, a high-strength steel for weight lightening also needs to be applied to chassis components playing important roles in improving the driving performance of vehicles. For achieving weight lightening of components, raising the strength of materials is essential, and also, it is important to secure the durability of components made of a high-strength steel in an environment in which fatigue load is repeatedly applied.
- In the case of arc-welding that is mainly used to secure strength upon assembly of vehicle chassis components, the components are overlapping-welded by depositing welding wires. Therefore, welding joints have geometric shapes inevitably, which act as repetitive fatigue stress concentrators (the notch effect) becoming breaking points, resulting in deterioration of the durability of the components. Accordingly, the advantage of applying the high-strength steel is lost.
- To improve the fatigue characteristics of a welded part, it is important to reduce an angle (also called a toe angle) of an end (also called a toe portion) of a welding joint which is a stress concentrator. Together with this, controlling the material and stress of the toe portion is also an important factor.
- Prior document has proposed a method of continuously hitting the ends of welding beads with a chipper (hitting pin) to form plastic-deformed areas, thereby improving fatigue characteristics by providing compression stress. However, in the cases of the proposed method of hitting with the chipper and typical technology of shot ball peening using air pressure, a target material may be deformed after peening. Therefore, applying them to a thin-film steel for vehicle is limited, and also, upon application to local areas such as the ends of welding beads, a defect such as micro-crack may be made in the welding beads having relatively high brittleness.
- The embodiments of the present disclosure are directed to providing a welding joint having excellent fatigue characteristics.
- Further, the embodiments of the present disclosure are directed to providing a method for manufacturing a welding joint having excellent fatigue characteristics, subjected to local remelting by irradiating a defocused laser thereon.
- One aspect of the present disclosure provides a welding joint having excellent fatigue characteristics, including: a
welded metal 20 formed by welding between a plurality ofsteel sheets 10 arranged in such a way to overlap each other at a connecting portion between the plurality ofsteel sheets 10; and aremolten portion 30 which is formed at an end of thewelded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater. - Also, according to an embodiment of the present disclosure, the
remolten portion 30 may be formed by irradiating a defocused laser 1 on the end of thewelded metal 20 to remelt the end of thewelded metal 20. - Also, according to an embodiment of the present disclosure, the surface layer of the
remolten portion 30 may have a bainite or martensite structure. - Also, according to an embodiment of the present disclosure, a toe angle θ of the
remolten portion 30 may be 30° or smaller. - Also, according to an embodiment of the present disclosure, fatigue strength of the welding joint may be 260 MPa or greater.
- Also, according to an embodiment of the present disclosure, fatigue strength of the welding joint may be 70% or more of fatigue strength of the
steel sheets 10. - Another aspect of the present disclosure provides a method for manufacturing a welding joint, including operation of irradiating a defocused laser 1 on an end of a
welded metal 20 formed by welding a connection portion between a plurality ofsteel sheets 10 arranged in such a way to overlap each other at the connecting portion to remelt the end of thewelded metal 20. - Also, according to an embodiment of the present disclosure, an output of the defocused laser 1 may be 6 kW, and movement speed of the defocused laser 1 may be 6 m/min.
- Also, according to an embodiment of the present disclosure, a defocus length L of the defocused laser 1 may be 15 to 20 mm.
- Also, according to an embodiment of the present disclosure, the defocus of the defocused laser 1 may cover the end of the
welded metal 20 and a part of a heat affected zone (HAZ) 11 of thesteel sheets 10 adjacent to the end of thewelded metal 20. - Also, according to an embodiment of the present disclosure, an irradiation angle of the defocused laser 1 may be 30 to 45°.
- In the welding joint having excellent fatigue characteristics according to the embodiments of the present disclosure, by irradiating a defocused laser on an end of a welded metal to increase the radiuses of curvature of the ends of welding beads through local remelting, the concentration of stress may be mitigated to improve fatigue strength of the welding joint.
- Also, by irradiating a defocused laser to perform remelting to thereby locally harden the surface layer of the ends of the welding beads, the fatigue strength of the welding joint may be improved.
-
-
FIG. 1 is a plan view for describing a welding joint having excellent fatigue characteristics, according to an embodiment of the present disclosure. -
FIG. 2 is a cross-sectional view of the welding joint shown inFIG. 1 , taken along line I-I' ofFIG. 1 . -
FIG. 3 is a cross-sectional view for describing a method for manufacturing a welding joint according to an embodiment of the present disclosure. -
FIG. 4 is a picture of a cross section of a welding joint according to an embodiment of the present disclosure, photographed by an optical microscope. -
FIG. 5 is a graph showing a hardness distribution of a welding joint according to an embodiment of the present disclosure. -
FIG. 6 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to an embodiment of the present disclosure, photographed by an optical microscope. -
FIG. 7 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to an embodiment of the present disclosure, photographed by an optical microscope. -
FIG. 8 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope. -
FIG. 9 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope. -
FIG. 10 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope. - A welding joint having excellent fatigue characteristics, according to an embodiment of the present disclosure, may include: a welded metal formed by welding between a plurality of steel sheets arranged in such a way to overlap each other at a connecting portion between the plurality of steel sheets; and a remolten portion which is formed at an end of the
welded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater. - Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to transfer the technical concepts of the present disclosure to one of ordinary skill in the art. However, the present disclosure is not limited to these embodiments, and may be embodied in another form. In the drawings, parts that are irrelevant to the descriptions may be not shown in order to clarify the present disclosure, and also, for easy understanding, the sizes of components are more or less exaggeratedly shown.
-
FIG. 1 is a plan view for describing a welding joint having excellent fatigue characteristics, according to an embodiment of the present disclosure.FIG. 2 is a cross-sectional view of the welding joint shown inFIG. 1 , taken along line I-I' ofFIG. 1 .FIG. 3 is a cross-sectional view for describing a method for manufacturing a welding joint according to an embodiment of the present disclosure.FIG. 4 is a picture of a cross section of a welding joint according to an embodiment of the present disclosure, photographed by an optical microscope.FIG. 5 is a graph showing a hardness distribution of a welding joint according to an embodiment of the present disclosure. - Referring to
FIGS. 1 to 5 , a welding joint according to an embodiment of the present disclosure, and a method for manufacturing the same will be described in detail. - The welding joint having the excellent fatigue characteristics, according to an embodiment of the present disclosure, may include a
welded metal 20 formed by welding between a plurality ofsteel sheets 10 arranged in such a way to overlap each other at a connecting portion between the plurality ofsteel sheets 10, and aremolten portion 30 which is formed at an end of thewelded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater. For example, the end of thewelded metal 20 may be an area of thewelded metal 20 adjacent to thesteel sheets 10. - For example, the
steel sheets 10 may be high-strength hot-rolled sheet steels having tensile strength of 780 MPa or higher and a thickness of 6 mmt or smaller. To apply a high-strength steel for weight lightening of a steel, the durability of a product made of such a high-strength steel needs to be secured. - In the case of arc-welding that is used to secure strength upon assembly of vehicle chassis components, the components are overlapping-welded by depositing welding wires. Therefore, welding joints have geometric shapes inevitably, which act as repetitive fatigue stress concentrators becoming breaking points, resulting in deterioration of the durability of the components.
- To improve the fatigue characteristics of a welded part, it may be important to reduce an angle (also called a toe angle) of an end (also called a toe portion) of a welding joint which is a stress concentrator. Together with this, controlling the material and stress of the toe portion may be also an important factor.
- To reduce the toe angle and to control the material and stress of the toe portion, the
remolten portion 30 may be formed at the end of the weldedmetal 20. - The
remolten portion 30 may be formed by irradiating a defocused laser on the end of the weldedmetal 20 to remelt the end of the weldedmetal 20. - As such, the end of the welded
metal 20, that is, the toe portion may be remolten through heat treatment using a defocused laser, and accordingly, theremolten portion 30 may have a toe angle θ of 30° or smaller. - When the toe angle θ of the
remolten portion 30 exceeds 30°, stress may be concentrated on theremolten portion 30, so that the fatigue strength is reduced to 100 MPa or smaller, resulting in a reduction of a fatigue life. - By reducing the toe angle of the
remolten portion 30, the radius of curvature of theremolten portion 30 may increase. Accordingly, since the radius of curvature of theremolten portion 30 acting as a repetitive fatigue stress concentrator increases, the concentration of stress onto theremolten portion 30 may be mitigated. - For example, the radius of curvature of the
remolten portion 30 may be greater than the radius of curvature of the weldedmetal 20. - By performing heat treatment of irradiating a defocused laser onto the end of the welded
metal 20, a surface layer of theremolten portion 30 may be partially hardened. - When the end of the welded
metal 20 is remolten to form theremolten portion 30, softening of a heat affected zone (HAZ) 11 may occur at thesteel sheets 10, etc. according to the remelting. - Since the defocused laser can minutely control its irradiating area, remelting may be locally generated, compared with cases in which different kinds of heat sources are used, thereby minimizing softening of a
HAZ 11 according to remelting. - As the surface layer of the
remolten portion 30 is hardened, a Vickers hardness value of the surface layer may be 200 HV or greater. - Herein, the surface layer of the
remolten portion 30 may mean an area within 800 µm from the surface of theremolten portion 30. When the Vickers hardness value of the surface layer of theremolten portion 30 is smaller than 200 HV, the welding joint may be easily fractured due to repetitive fatigue stress. -
FIG. 5 is a graph showing a hardness distribution of the welding joint. Referring toFIG. 5 , it is confirmed that a hardness value of the surface layer of theremolten portion 30 is equal to or greater than 200 Hv. - The surface layer of the
remolten portion 30 subject to the heat treatment may have a bainite or martensite structure. For example, a bainite or martensite structure may appear within 800 µm from the surface of theremolten portion 30. - Accordingly, the fatigue strength of the welding joint may be 260 MPa or greater.
- For example, the fatigue strength of the welding joint may be 70% or more of the fatigue strength of the
steel sheets 10. That is, by securing the fatigue strength of the welding joint to a level that is maximally similar to that of a steel sheet that is a base material, an application range of a high-strength hot-rolled sheet steel for thinning and weight lightening of components such as vehicle chassis components may be expanded. - The welding joint according to an embodiment of the present disclosure may be manufactured through operation of irradiating a defocused laser on an end of the welded
metal 20 formed by welding a connecting portion between the plurality ofsteel sheets 10 arranged in such a way to overlap each other at the connecting portion to remelt the end of the welded metal 2. - A defocused laser 1 may irradiate a laser on the end of the welded
metal 20. - For example, the defocused laser 1 may be a CO2 solid-state laser.
- However, the output of the defocused laser 1 may be limited to this, and for example, the output of the defocused laser 1 may be 6 kW.
- For example, head movement speed of the defocused laser 1 may be 4 to 6 m/min.
- When the head movement speed of the defocused laser 1 is lower than 4 m/min, the end of the welded
metal 20, that is, the surface layer material of theremolten portion 30 may be not easily hardened. When the head movement speed of the defocused laser 1 is higher than 6 m/min, a sufficient melting rate for remelting at the end of the weldedmetal 20 may be not secured so that a desired toe angle of theremolten portion 30 may be not secured. - For example, a defocus length L of the defocused laser may be 15 to 20 mm.
- For example, the defocus of the defocused laser may cover the end of the welded
metal 20 and a part of theHAZ 11 of thesteel sheets 10 adjacent to the end of the weldedmetal 20. - When the defocus length L of the defocused laser is shorter than 15 mm, there is a problem that the defocused laser cannot remelt all of the end of the welded
metal 20 and theHAZ 11 of thesteel sheets 10 formed by welding. When the defocus length L of the defocused laser is longer than 20 mm, there is a problem that a heat input for remelting is insufficient. - For example, an irradiation angle of the defocused laser may be 30 to 45°.
- When the irradiation angle of the defocused laser is smaller than 30°, a sufficient melting rate for remelting at the end of the welded
metal 20 may be not secured so that a desired toe angle of theremolten portion 30 may be not secured. When the irradiation angle of the defocused laser is greater than 45°, a sufficient melting rate may be not secured at alower steel sheet 10A of the overlapping joint so that the surface layer material may be not easily hardened. -
FIG. 6 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to an embodiment of the present disclosure, photographed by an optical microscope.FIG. 7 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to an embodiment of the present disclosure, photographed by an optical microscope.FIG. 8 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope.FIG. 9 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope.FIG. 10 is a picture of a cross section of a welding joint manufactured by a welding joint manufacturing method according to a comparative example of the present disclosure, photographed by an optical microscope. - Hereinafter, the present disclosure will be described in more detail with reference to
FIGS. 6 to 10 and embodiments. - PO (Pickled & Oiled) 780HB steel sheets are arranged in such a way to overlap each other, and then a connecting portion between the PO (Pickled & Oiled) 780HB steel sheets is arc-welded to form a welded metal. Thereafter, a defocused laser is irradiated onto a toe portion of the welded metal to remelt the toe portion of the welded metal. At this time, an output of the defocused laser is 6kW, laser head movement speed is 4 m/min, a defocus length is 15 mm, and an irradiation angle is 30°.
FIG. 6 is a picture of the cross section of a welding joint according to the embodiment 1, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the embodiment 1 are measured and written on Table 1 below. - A welding joint is manufactured according to the same method as in the embodiment 1.
FIG. 7 is a picture of the cross section of a welding joint according to the embodiment 2, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the embodiment 2 are measured and written on Table 1 below. - A welding joint is manufactured according to the same method as in the embodiment 1, except that the output of the defocused laser is 6 kw, the laser head movement speed is 4 m/min, the defocus length is 10 mm, and the irradiation angle is 20°.
FIG. 8 is a picture of the cross section of the welding joint according to the comparative example 1, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the comparative example 1 are measured and written on Table 1 below. - A welding joint is manufactured according to the same method as in the embodiment 1, except that the output of the defocused laser is 6 kw, the laser head movement speed is 2 m/min, the defocus length is 10 mm, and the irradiation angle is 20°.
FIG. 9 is a picture of the cross section of the welding joint according to the comparative example 2, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the comparative example 2 are measured and written on Table 1 below. - A welding joint is manufactured according to the same method as in the embodiment 1, except that the output of the defocused laser is 6 kw, the laser head movement speed is 2 m/min, the defocus length is 15 mm, and the irradiation angle is 30°.
FIG. 10 is a picture of the cross section of the welding joint according to the comparative example 3, photographed by an optical microscope. The manufacturing conditions and fatigue strength of the welding joint according to the comparative example 3 are measured and written on Table 1 below. - PO (Pickled & Oiled) 780HB steel sheets are arranged to overlap each other, a connecting portion between the PO (Pickled & Oiled) 780HB steel sheets is subject to TIG arc welding to form a welded metal, and then a toe portion of the welded metal is remolten by TIG arc, instead of a defocused laser, thereby completing welding. The manufacturing conditions and fatigue strength of a welding joint according to the comparative example 4 are measured and written on Table 1 below.
[Table 1] Laser Output (kW) Movement Speed (m/min.) Defocus Length (mm) Irradiation Angle (°) Fatigue Strength (MPa) Embodiment 1 6 4 15 30 267 Embodiment 2 6 4 15 30 273 Comparative Example 1 6 4 10 20 252 Comparative Example 2 6 2 10 20 Nonmeasurable Comparative Example 3 6 2 15 30 Nonmeasurable Comparative Example 4 - - - - 217 - Table 2 is a table for describing a method for measuring the fatigue strength of the welding joint according to the embodiment 2.
[Table 2] Maximum Load (kN) Maximum Strength (MPa) Number of Repetitions (cycles) Fracture or Non-fracture Test 1 25.0 268 2,000,000 Non-fracture Test 2 25.5 273 2,000,000 Non-fracture (Fatigue Strength) Test 3 26.0 277 92,586 Fracture - Referring to Table 2, the welding joint is not fractured even at the repetition number of 2,000,000 cycles under the maximum load of 25.5kN, and at this time, the maximum strength is 273 MPa. The welding joint is fractured at 92,586 cycles under the maximum load of 26.0 kN increased by 0.5 kN. At this time, the maximum strength is 277 MPa. Accordingly, the fatigue strength of the welding joint according to the embodiment 2 is 273MPa.
- Table 3 is a table for describing a method for measuring the fatigue strength of the welding joint according to the comparative example 4.
[Table 3] Maximum Load (kN) Maximum Strength (MPa) Number of Repetitions (cycles) Fracture or Non-fracture Test 2-1 20.0 217 2,000,000 Non-Fracture (Fatigue Strength) Test 2-2 20.5 219 1,227,207 Fracture Test 2-3 21.0 224 756,193 Fracture - Referring to Table 3, the welding joint is not fractured even at the repetition number of 2,000,000 cycles under the maximum load of 20.0 kN, and at this time, the maximum strength is 217 MPa. Accordingly, it is confirmed that the fatigue strength of the welding joint according to the comparative example 4 is 217 MPa.
- As described above, since the welding joint according to an embodiment of the present disclosure secures fatigue strength of 260 MPa, an application range of a high-strength hot-rolled sheet steel for thinning and weight lightening of components such as vehicle chassis components may be expanded.
- As needs for a high-strength steel and a high-burring steel are steadily increasing, a welding technique including the welding joint having excellent fatigue characteristics according to the embodiments of the present disclosure may be used in various fields such as industrial sites.
Claims (11)
- A welding joint having excellent fatigue characteristics, comprising:a welded metal 20 formed by welding between a plurality of steel sheets 10 arranged in such a way to overlap each other at a connecting portion between the plurality of steel sheets 10; anda remolten portion 30 which is formed at an end of the welded metal 20 and whose surface layer has a Vickers hardness value of 200 Hv or greater.
- The welding joint of claim 1, wherein the remolten portion 30 is formed by irradiating a defocused laser 1 on the end of the welded metal 20 to remelt the end of the welded metal 20.
- The welding joint of claim 1, wherein the surface layer of the remolten portion 30 has a bainite or martensite structure.
- The welding joint of claim 1, wherein a toe angle θ of the remolten portion 30 is 30° or smaller.
- The welding joint of claim 1, wherein fatigue strength of the welding joint is 260 MPa or greater.
- The welding joint of claim 1, wherein fatigue strength of the welding joint is 70% or more of fatigue strength of the steel sheets 10.
- A method for manufacturing a welding joint, comprising operation of irradiating a defocused laser 1 on an end of a welded metal 20 formed by welding a connection portion between a plurality of steel sheets 10 arranged in such a way to overlap each other at the connecting portion to remelt the end of the welded metal 20.
- The method of claim 7, wherein an output of the defocused laser 1 is 6 kW, and movement speed of the defocused laser 1 is 4 to 6 m/min.
- The method of claim 7, wherein a defocus length L of the defocused laser 1 is 15 to 20 mm.
- The method of claim 9, wherein the defocus of the defocused laser 1 covers the end of the welded metal 20 and a part of a heat affected zone (HAZ) 11 of the steel sheets 10 adjacent to the end of the welded metal 20.
- The method of claim 7, wherein an irradiation angle of the defocused laser 1 is 30 to 45°.
Applications Claiming Priority (2)
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KR1020160067841A KR101720087B1 (en) | 2016-06-01 | 2016-06-01 | Welded joint having excellent fatigue property and method of manufacturing the same |
PCT/KR2017/005527 WO2017209459A1 (en) | 2016-06-01 | 2017-05-26 | Welding joint having excellent fatigue characteristics and method for manufacturing same |
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EP3467129A4 EP3467129A4 (en) | 2019-04-10 |
EP3467129A1 true EP3467129A1 (en) | 2019-04-10 |
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EP17806952.2A Ceased EP3467129A1 (en) | 2016-06-01 | 2017-05-26 | Welding joint having excellent fatigue characteristics and method for manufacturing same |
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US (1) | US20190194776A1 (en) |
EP (1) | EP3467129A1 (en) |
JP (1) | JP2019516556A (en) |
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DE102020119395A1 (en) * | 2019-07-24 | 2021-01-28 | Shiloh Industries, Inc. | Sheet metal assembly with post-treated welded joint |
EP4039396A4 (en) * | 2019-10-04 | 2022-12-14 | Nippon Steel Corporation | Welded joint and automobile component |
JP7376779B2 (en) * | 2019-11-07 | 2023-11-09 | 日本製鉄株式会社 | Welded joints and auto parts |
CN116135398A (en) * | 2021-11-16 | 2023-05-19 | 通快(中国)有限公司 | Improvement method, welding method, processing system, control device, and program product |
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KR970033384A (en) * | 1995-12-23 | 1997-07-22 | 이우복 | Welding method using laser beam |
JP2002018583A (en) * | 2000-07-07 | 2002-01-22 | Hitachi Zosen Corp | Method of laser beam welding |
JP4173999B2 (en) * | 2002-04-01 | 2008-10-29 | 新日本製鐵株式会社 | Fillet welding method and fillet welded joint of steel plate with excellent fatigue strength of welded part |
JP2004148333A (en) * | 2002-10-29 | 2004-05-27 | Nippon Steel Corp | Method for enhancing fatigue strength of lap fillet joint |
JP2007105754A (en) * | 2005-10-13 | 2007-04-26 | Daihen Corp | Laser irradiation arc welding method |
JP5450293B2 (en) * | 2010-07-01 | 2014-03-26 | 株式会社神戸製鋼所 | Fillet welded joint and gas shielded arc welding method |
EP2839918B1 (en) * | 2012-04-17 | 2020-11-18 | Nippon Steel Corporation | Fillet arc welded joint and method for forming same |
KR20130124729A (en) * | 2012-05-07 | 2013-11-15 | 주식회사 포스코 | Laser welding method for high strength steels |
JPWO2013179614A1 (en) * | 2012-05-29 | 2016-01-18 | Jfeスチール株式会社 | Laser-arc hybrid welding method |
JP5985901B2 (en) | 2012-06-25 | 2016-09-06 | Jfeスチール株式会社 | Welded joint and method for forming the same |
EP2926939B1 (en) * | 2012-11-29 | 2017-11-15 | Nippon Steel & Sumitomo Metal Corporation | Method for forming fillet arc welding joint and fillet arc welding joint |
CN103341695A (en) * | 2013-06-27 | 2013-10-09 | 西安交通大学 | Method for improving mechanical property of hardened and tempered low-alloy high-strength steel GMAW connector |
US10092982B2 (en) * | 2015-02-13 | 2018-10-09 | Nippon Steel & Sumitomo Metal Corporation | Fillet welded joint and method of production of same |
WO2017018492A1 (en) * | 2015-07-28 | 2017-02-02 | 新日鐵住金株式会社 | Fillet arc welding joint and method for manufacturing same |
CN105397337B (en) * | 2015-12-18 | 2018-02-27 | 中国航空工业集团公司北京航空制造工程研究所 | A kind of method that using microwave absorbing coating Titanium Alloy Welds are carried out with laser post-treatment weldering |
-
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- 2017-05-26 WO PCT/KR2017/005527 patent/WO2017209459A1/en unknown
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EP3467129A4 (en) | 2019-04-10 |
WO2017209459A1 (en) | 2017-12-07 |
CN109196126A (en) | 2019-01-11 |
KR101720087B1 (en) | 2017-03-28 |
CN109196126B (en) | 2020-09-25 |
JP2019516556A (en) | 2019-06-20 |
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